title: Appropriate goal level for 25-hydroxyvitamin D in cystic fibrosis.
abstract: BACKGROUND
  Vitamin D deficiency is common in patients with cystic fibrosis (CF), and guidelines recommend 25-hydroxyvitamin D (25OHD) levels ≥ 30 ng/mL. This threshold was selected because serum parathyroid hormone (PTH) rises in healthy individuals when the 25OHD level falls below 30 ng/mL. PTH levels > 50 pg/mL are associated with an increased risk of bone loss. However, the relationship between 25OHD and PTH has not been studied in CF. We sought to determine the appropriate goal 25OHD level in patients with CF by identifying the level below which the risk of PTH > 50 pg/mL begins to increase.
  
  
  METHODS
  Levels of 25OHD and PTH in 216 individuals with CF were collected prospectively. Individuals with 25OHD < 30 ng/mL were treated with vitamin D2, and levels were reevaluated.
  
  
  RESULTS
  Mean 25OHD level was 25.7 ± 12.4 ng/mL, and mean PTH level was 46.7 ± 25.9 pg/mL. In 63% of individuals, 25OHD level was < 30 ng/mL, and in 38.0% it was ≤ 20 ng/mL. Low 25OHD levels were significantly associated with elevated PTH levels, with a mean PTH of 53.1 ± 29.8 pg/mL for 25OHD level 0 to 19 ng/mL; 51.1 ± 30.7 pg/mL for 25OHD level 20 to 29 ng/mL; 38.4 ± 16.4 pg/mL for 25OHD level 30 to 39 ng/mL; and 37.2 ± 16.4 pg/mL for 25OHD level ≥ 40 ng/mL (P = .006). We assessed the sensitivity of different 25OHD thresholds to identify individuals meeting the goal of a PTH level < 50 pg/mL to reduce the risk of bone loss. To obtain 90% sensitivity, a 25OHD level ≥ 35 ng/mL was required. Strikingly, 23% of individuals with 25OHD levels 30 to 34 ng/mL still had a PTH level > 50 pg/mL. This decreased to 14% for 25OHD level ≥ 35 ng/mL.
  
  
  CONCLUSIONS
  Inadequate serum 25OHD levels are common in adults with CF and are associated with elevated PTH levels. Aiming to maintain 25-OHD levels ≥ 35 ng/mL in individuals with CF decreases the risk of having a PTH level associated with secondary hyperparathyroidism and bone loss.
authors.affiliation: Division of Pulmonary and Critical Care Medicine, Department of Medicine, Johns Hopkins University, Baltimore, MD. Electronic address: nwest5@jhmi.edu.
authors.forename: Natalie E
authors.initials: NE
authors.lastname: West
authors.affiliation: Division of Pulmonary and Critical Care Medicine, Department of Medicine, Johns Hopkins University, Baltimore, MD.
authors.forename: Noah
authors.initials: N
authors.lastname: Lechtzin
authors.affiliation: Division of Pulmonary and Critical Care Medicine, Department of Medicine, Johns Hopkins University, Baltimore, MD.
authors.forename: Christian A
authors.initials: CA
authors.lastname: Merlo
authors.affiliation: Division of Pulmonary and Critical Care Medicine, Department of Medicine, University of Pennsylvania, Philadelphia, PA.
authors.forename: Jason B
authors.initials: JB
authors.lastname: Turowski
authors.affiliation: Department of Nutrition, Johns Hopkins University, Baltimore, MD.
authors.forename: Marsha E
authors.initials: ME
authors.lastname: Davis
authors.affiliation: Division of Pulmonary and Critical Care Medicine, Department of Medicine, Johns Hopkins University, Baltimore, MD.
authors.forename: Meghan Z
authors.initials: MZ
authors.lastname: Ramsay
authors.affiliation: Division of Pulmonary and Critical Care Medicine, Department of Medicine, Johns Hopkins University, Baltimore, MD.
authors.forename: Sharon L
authors.initials: SL
authors.lastname: Watts
authors.affiliation: Department of Medicine, Vanderbilt University, Nashville, TN.
authors.forename: Shane P
authors.initials: SP
authors.lastname: Stenner
authors.affiliation: Division of Pulmonary and Critical Care Medicine, Department of Medicine, Johns Hopkins University, Baltimore, MD.
authors.forename: Michael P
authors.initials: MP
authors.lastname: Boyle
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D002118:Calcium / Q000097:blood; D003550:Cystic Fibrosis / Q000097:blood* / Q000150:complications; D019587:Dietary Supplements; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D010281:Parathyroid Hormone / Q000097:blood; D014807:Vitamin D / Q000008:administration & dosage / Q000031:analogs & derivatives* / Q000097:blood; D014808:Vitamin D Deficiency / Q000150:complications / Q000628:therapy; D055815:Young Adult
chemical_list: D010281:Parathyroid Hormone; D014807:Vitamin D; C104450:25-hydroxyvitamin D; D002118:Calcium